Lubricating fluids, such as the motor oil used in internal combustion engines, undergo a significant decrease in their lubricating qualities over the course of their use. This deterioration can be due to many different factors, among these being contamination from other chemicals such as engine coolant, breakdown of chemical additives in the oil, and the increase in oil viscosity due to harsh operating conditions. In order to prevent undue wear and damage to the machines they are intended to protect, lubricants must be replaced before their lubricating qualities diminish to an unacceptable level. Changing the motor oil at a fixed milage or time interval can be very wasteful, since the interval must be based on a conservatively short estimate of the useful life of the oil. Many methods have been proposed in which an electrical characteristic of the lubricant is measured and used to evaluate its lubricating properties.
U.S. Pat. No. 2,752,566 describes a method of checking for chemical and solid particulate contamination in an oil sample by measuring the resistance of the sample to electrical current. Chemical contamination is evaluated by placing electrodes in the sample with a gap distance well in excess of the maximum expected working clearances between parts in the engine. Solid particulate contamination is evaluated with electrodes set at a gap distance approximately equal to or slightly less than the normal working clearances.
U.S. Pat. No. 4,029,554 describes a method in which electrodes made of dissimilar materials are placed in an oil sample, and oxide byproducts present in the oil cause an oxidation reaction to take place with the electrodes. This oxidation reaction produces a corresponding voltage output, and this voltage output is amplified and measured. The greater the amount of oxidation byproducts present in the oil sample, the greater the voltage produced.
U.S. Pat. No. 5,028,144 describes an apparatus for measuring the amount of moisture contamination in brake fluid. This is accomplished by measuring the electrical conductivity of the fluid and comparing it with benchmark values of conductivity obtained from fluid samples with known moisture contents.
U.S. Pat. No. 4,686,857 describes a method in which a short duration DC pulse is applied to two electrodes submerged in the lubricant under test, and the resulting transient response current is detected. Because a direct current is used, the molecules of various contaminants in the oil undergo electrolytic dissociation resulting in a rapid decrease in current flow as the fluid polarizes. In one form of this device, the fluid under test is heated or cooled to a prescribed temperature so that the test results do not show any current variation relating to differences in temperature.
Because of their relatively high cost and complexity, none of these devices have achieved any substantial degree of commercial success and there accordingly exists a need for a reliable low cost, simple, method and apparatus for measuring the quality of lubricating oil and the like.